Chemical discovered that causes plant tumors

CORVALLIS, Ore. - Researchers in Oregon and elsewhere have discovered a new class of chemicals they call "bruchins" which, even at extraordinarily low levels of exposure, signal a plant to form a benign tumor, apparently as part of the plant's defensive mechanism against an insect pest.

The study, done by scientists from Oregon State University, the U.S. Department of Agriculture and a private company, will be published Tuesday in the Proceedings of the National Academy of Sciences. There is no immediate commercial application that's apparent about this discovery, the researchers said, but the unusual capabilities of this group of chemicals is highly interesting. It's also the first chemical of this type ever identified that can induce tumor formation in plants, they said.

"This is an extremely active new class of chemicals that has a clear impact on plant growth regulation," said William Proebsting, a professor of horticulture at OSU. "We could observe its biological impact on the plant at levels so low we couldn't even physically detect the chemical. Whenever you find a new group of chemicals with this level of activity you get a little excited. And this is also the type of basic discovery that often sets the stage for later applications."

In this research, the scientists studied the interaction of pea plants with pea weevils, a tiny insect that's smaller than a ladybug but is one of the worst insect pests of peas. In order to reproduce, the pea weevil needs to lay its eggs on a pea pod. But the pea plant has apparently developed an innovative and effective response to this insect attack.

When the egg is laid, the pea plant detects in the pea weevil a type of chemical, which the Oregon researchers identified and are calling bruchins (in reference to the scientific classification, bruchid) to which pea weevils belong.

This chemical triggers the pea pod to begin a process of cellular division that actually forms a small tumor, lifting the pea weevil egg up and away from the pea pod. In this position it helps to prevent the emerging weevil larvae from burrowing into the pea pod. It may also fall off, dry out or be eaten by predators, the researchers said, and it reduces the chance that the pea weevil will actually succeed in infesting a pea.

"It's pretty obvious this is a natural defense mechanism that the pea plants have developed, and for purposes of survival in nature, it works pretty well," said Robert Doss, a plant physiologist with the USDA Agricultural Research Service and OSU Department of Horticulture. Doss was a co-principal investigator on the study along with Proebsting and James Oliver, a chemist with the Agricultural Research Service in Beltsville, Md.

All practical applications aside, the scientists said, a chemical such as this is a wonder of nature.

"This may or may not lead to something that is useful in agriculture or elsewhere, but it always helps to understand the relationship between organisms," Doss said. "And anything we find that relates to the control of cell division is pretty important."

The new bruchin chemicals not only are active at extremely low levels, the researchers found, but the reaction of the pea plants to them is highly precise. When artificially applied to pea plants, tumors began to form almost immediately on the pea pods - where they would have a useful role in nature in repelling pea weevils - but there was no effect on most other parts of the same plant.

Earlier researchers had identified a gene in the pea plant that is required for formation of these tumors. Now the Oregon researchers and their collaborators have discovered the key chemical produced by an insect pest that triggers the process.

A number of other plants have similar "galls," or plant tumors on them, the researchers said. The round scabs often found on oak leaves are one example. But this process often actually works to the benefit of the insects that interact with these plants, Doss said, so in various natural systems it's not clear that there is any single evolutionary explanation for the formation of these tumors.

The discovery of basic plant growth mechanisms can have important implications, the scientists said. When a type of plant hormone called "auxin" was first identified years ago that played a role in cell elongation, there was no obvious use for the findings. Later, this basic information led to the creation of root-stimulating hormones, new types of herbicides and other products.